Optical cable (fiber) is utilized in a variety of applications to carry information that is modulated onto a beam of light. In some applications significant portions of the optical cable are in an environment where the optical cable is not normally subjected to substantial physical vibration. In other applications optical cable is utilized to carry information between two locations where the optical cable is subject to substantial movement, flexing or vibration.
FIG. 1 illustrates an example of an application where the optical cable 10 is subject to substantial movement. In this application phase information is transmitted from a towed array 12 of acoustic sensors towed behind a watercraft 14 to an optical receiver 16 in the watercraft. An optical sensor 18 may use a Mach-Zehnder interferometer as described in U.S. Pat. No. 5,448,058 and incorporated herein by reference. One or more light beams carrying the sensed information are transmitted to the optical receiver by one or more optic fibers in optical cable 10. A light source, e.g. a laser, is associated with the receiver 16 and transmits a light beam on one fiber of the cable 10 as an input to the sensor which is typically a passive device such as an interferometer. Another cable 20, e.g. a steel cable, absorbs the forces required to pull the sensor array through the water. In this environment the optical cable experiences substantial physical movement, i.e. low-frequency vibration typically below 50 hertz known as tow cable strum.
Movement of an optical cable carrying information encoded on a light beam can give rise to a change in the state of polarization of the transmitted light. Unwanted changes in the state of polarization of the transmitted light such as due to tow cable strum are manifest as polarization noise at the optical receiver.
Polarization diversity detection has been utilized in an optical receiver to overcome polarization fading. The desired signal can disappear at the optical receiver due to polarization fading such as when the two light beams in a Mach-Zehnder interferometer are orthogonal. A polarization diversity receiver is described in U.S. Pat. No. 5,852,507, which is incorporated herein by reference. Polarization optics or masks have been employed at the receiver adjacent to the detector elements to achieve polarization diversity reception.
However, rapidly changing states of polarization of the transmitted light, such as due to tow cable strum, can create sufficient polarization noise at the receiver to substantially impair the detection of the transmitted optical signal. Polarization masks or shifters used at the receiver prior to signal detection are useful in helping to eliminate signal fading due to polarization crossing but also induce unwanted polarization noise at the receiver when the light beam carrying the signal is subjected to undesired changes of polarization state. Thus, there exists a need for an improved optical system that can take advantage of polarization diversity while minimizing difficulties with polarization noise.